This invention relates to apparatus and processes for carbon black production, and especially to systems for producing carbon black of negative tint residual.
Carbon black reactors and methods of producing carbon black are well known in the prior art. Most modern carbon black reactors comprise a longitudinally extending reactor tunnel of refractory material of generally circular cross-sectional configuration defining combustion or precombustion zones, reaction zones and quench zones in contiguous axial alignment. A hydrocarbon fluid fuel, usually natural gas or fuel oil, is burned in the combustion zone tunnels with a stream of process air furnished by a blower, usually using excess air over stoichiometric requirements. The hot gases produced by the combustion of the fuel are directed, usually tangentially, into the precombustion zone and are admixed with injected carbon black feedstock, usually a heavy aromatic oil, which is usually injected as a spray of fine droplets. As the oil enters the flowing hot gases, it undergoes a series of reactions to produce the product carbon black, partially in the precombustion zone and partially in the reaction zone. The admixture of produced carbon black and hot gases (called smoke) is quenched, usually by water spray and/or by cooled reaction effluent, in the quench zone to a temperature below that at which further carbon black formation occurs. The produced carbon black is finally separated from the gases and is recovered as the carbon black product.
The physical and chemical changes that occur in the formation of carbon black from the feedstock oil are very complex. Heat is transferred rapidly to the oil droplets and vapor from the hot combustion gases, hot refractory, and combustion of a portion of the oil by oxygen which is added with the excess air used to combust the fuel. Under these conditions, feedstock oil molecules are dehydrogenated, forming carbon nuclei. The nuclei grow in size to form particles or nodules which aggregate into cluster-like agglomerates, the degree of aggregation being commonly referred to as "structure". These two properties, "structure" and particle or nodule size, are of paramount importance in the production of carbon black since they determine to a large extent the properties of manufactured articles containing carbon black. Other properties include pitting due to aftertreatment and surface chemistry.
The particle size and "structure" of produced carbon black may be measured by a variety of tests. Particle size is normally determined by measuring the surface area of the produced carbon black particles. Surface area may be determined directly or indirectly. The "tint" test, an indirect determination, measures the ability of a produced carbon black to cover the surface of a finely divided zinc pigment as compared with a standard carbon black. Adsorptive tests are used to directly measure the surface area of produced carbon black. Included among these adsorptive tests are the nitrogen adsorption (N.sub.2 SA) test, the iodine test, and the cetyltrimethyl ammonium bromide (CTAB) test. The CTAB molecules are too large to enter pores in the carbon black particles that may be entered by smaller nitrogen molecules. The difference between the N.sub.2 SA test results and the CTAB test results is a measure of the porosity or pitting of the carbon black nodules which is another important carbon black property.
"Structure" is measured by the amount of dibutyl phthalate (DBP) absorbed by a given sample of produced carbon black. The sample of carbon black may be tested for DBP absorption both before and after being subjected to an exact crushing pressure, since the "structure" of produced carbon black drops to a lower constant value upon mechanical working. Structure as measured after such compression is a better measure of the structure of the black, such as would be encountered by mixing carbon black with other raw materials to produce manufactured articles. The test is known as the DBP test when it is performed without crushing the carbon black. When the carbon black is crushed before absorption, the test is known as the "24M4 DBP" test, or compressed DBP test, since the sample is subjected to 24,000 pounds per square inch gage of pressure four times before the dibutyl phthalate is added.
It has been disclosed in U.S. Pat. No. 4,071,496 issued Jan. 31, 1978, assigned to Phillips Petroleum Company, that the incorporation of a carbon black with a negative tint residual, preferably below minus 6, into a rubber composition causes the hysteresis property, or heat buildup, of such a rubber composition to be significantly lower than that of a rubber composition incorporating a carbon black with a higher tint residual, while the abrasion resistance of such rubber compositions incorporating these different carbon blacks remains essentially the same. Great importance has, therefore, been placed upon the discovery of methods and apparatus that are capable of producing carbon black having negative tint residual. For example, it has been found that carbon black of negative tint residual in a range below -6 is very desirable in the production of rubber compositions used in automobile tires because such tires exhibit low heat buildup under road conditions while retaining high abrasion resistance. The production of negative tint residual carbon black has, therefore, become increasingly important.
Tint residual is the difference between the measured tint and the tint calculated from the structure (24M4 DBP), nitrogen surface area (N.sub.2 SA) and CTAB surface area in accordance with a specific formula. EQU TR=T-[56.0+1.057(CTAB)-0.002745(CTAB).sup.2 - 0.2596(24M4 DBP)-0.201 (N.sub.2 SA-CTAB)]
In this formula the abbreviations used have the following meanings and the properties are measured as described:
TR: This is the tint residual. PA1 24M4 DBP: This is a measure of structure of the carbon black in cubic centimeters per 100 grams and is measured in accordance with ASTM 3493. This property is also referred to as compressed DBP. PA1 CTAB: This is the surface area of the carbon black measured in accordance with ASTM D3765-79, square meters per gram. PA1 N.sub.2 SA: This is the surface area of the carbon black in square meters per gram, and is measured using nitrogen in accordance with ASTM D 3037. PA1 T: This is the tint or tinting strength of the carbon black relative to the industrial reference black IRB No. 3, which has the tint value of 100. Tint is measured in accordance with ASTM 3265. The surface area, described above as CTAB and N.sub.2 SA, is an inverse measurement of the nodule size of the carbon black. The structure of the carbon black, expressed above as 24M4 DBP, is a measure of the complexity of the individual carbon black aggregates or of the number of nodules "fused" together in one carbon black aggregate. PA1 CTAB: 73 to 140 square meters/gram PA1 24M4 DBP: 67 to 111 cc/100 g PA1 (N.sub.2 SA-CTAB): 23 square meters/gram or less
Among the carbon blacks defined by the formula given above, those that are further characterized by having surface area and structure properties within the following ranges are particularly preferred:
All these properties, namely Tint, CTAB, 24M4 DBP and N.sub.2 SA, are defined and measured by ASTM procedures, as disclosed above.
Additional information relating tint residual to aggregate size distribution and nodule size distribution is detailed by Stacy, et al in "Effect of Carbon Black Structure Aggregate Size Distribution on Properties of Reinforced Rubber," Rubber Chemistry and Technology, Volume 48, No. 4, September-October, 1975, pages 538 through 547.